plasma-workspace: fix systemd unit detection

Our units are links in systemd unit directory, which reports state `linked` or
`linked-runtime`. This fixes systemdBoot config and the option
`services.xserver.desktopManager.plasma5.runUsingSystemd` for plasma 5.23.

pkgs/desktops/plasma-5/plasma-workspace/0003-startkde-unit-detection.patch

···
+diff --git a/startkde/startplasma.cpp b/startkde/startplasma.cpp
+index a17967b47..6e6141b48 100644
+--- a/startkde/startplasma.cpp
++++ b/startkde/startplasma.cpp
+@@ -431,7 +431,11 @@ bool hasSystemdService(const QString &serviceName)
+                                               QStringLiteral("/org/freedesktop/systemd1"),
+                                               QStringLiteral("org.freedesktop.systemd1.Manager"),
+                                               QStringLiteral("ListUnitFilesByPatterns"));
+-    msg << QStringList({QStringLiteral("enabled"), QStringLiteral("static")}) << QStringList({serviceName});
++    msg << QStringList({QStringLiteral("enabled"),
++                        QStringLiteral("static"),
++                        QStringLiteral("linked"),
++                        QStringLiteral("linked-runtime")});
++    msg << QStringList({serviceName});
+     QDBusReply<QList<QPair<QString, QString>>> reply = QDBusConnection::sessionBus().call(msg);
+     if (!reply.isValid()) {
+         return false;

pkgs/desktops/plasma-5/plasma-workspace/default.nix

···
   patches = [
     ./0001-startkde.patch
     ./0002-absolute-wallpaper-install-dir.patch
+    ./0003-startkde-unit-detection.patch
     # Included in 5.23.2
     (fetchpatch {
       name = "ignore-placeholder-screens";